1. Field of the Invention
The invention relates to a method and device for evaluating received signals acquired during a non-destructive ultrasonic wave test on a test body.
2. Description of the Prior Art
Electromagnetic ultrasonic probes, EMUS probes for short, are used in a manner known per se for the non-destructive examination or measurement of test objects, which possess at least electrically conductive and ferromagnetic material parts, and are capable of generating and also detecting, free from coupling means, elastic waves inside a test object. From the large number of different embodiments of such probes, reference is made to DE 26 55 904 B2 and DE 195 43 481 C2 as being representative.
The working principle underlying the EMUS probes is based on the provision of an electric coil with a predetermined geometry and number of turns, which is acted upon by a high frequency (HF) current pulse or also a burst signal, the mid-frequency whereof corresponds to the frequency of ultrasound waves that is generated in a desired manner inside the test object. The HF current pulse thus induces within the skin depth of an electrically conductive (preferably homogeneous medium located close to the electric coil) electric eddy currents the two-dimensionally extending distribution whereof within the skin depth being mirror-inverted to the geometry of the coil area. If a static or almost static magnetic field is superimposed on the eddy currents induced inside the medium, with magnetic field lines orientated parallel to the material surface or normal to the latter, spatial and temporally periodic elastic displacements in the form of elastic waves in the frequency range of the ultrasound are generated inside the medium. The displacements are capable of being duly propagated inside the medium.
Conversely, an elastic wave arriving at the site of the electric coil generates, in the presence of a stationary or almost stationary magnetic field, an electrical field proportional to the displacement speed and the magnetic induction, as a result of which a proportional electric voltage is induced in the coil. This mechanism for generating electrical signals from elastic wave pulses can be used for the detection of ultrasound waves by using EMUS probes. However, the signal levels of the voltage pulses generated in this way lie in the μV range, so that a strong and low-noise pre-amplification is required in order to obtain, as far as possible, signal levels that can be evaluated. Furthermore, the pre-amplified signal levels are subjected to electrical filtering with as narrow a band as possible in order to improve the signal-to-noise ratio.
Due to its electrical inductivity, however, the coil is also capable of receiving electromagnetic signals other than those which, as described above, originate from the elastic wave pulses. In addition, there is the fact that the impedance of the coil is designed with as high a resistance as possible in order to generate voltage signal levels as high as possible. Both factors mean that EMUS probes also, apart from detecting voltage signals originating from ultrasound waves, detect signals which originate from externally inductively acting electromagnetic signal sources. The latter run through the same amplification and, as the case may be, filter chain as the ultrasound signals and cannot be readily distinguished from the ultrasound signals.